Symptomatic cervical disc disease is commonly treated by an extensive anterior excision of the affected disc and its replacement with a similarly shaped iliac bone graft. Removal of the sick disc allows the interspace, the area previously occupied by the disc between two adjacent vertebrae, to collapse. Since the graft depends on proper shaping and an interference fit for its stability, the interspace must be distracted beyond its original height to assure an adequately tight fit.
Distraction of the interspace is presently achieved in one of three ways:
1. Traction--This generally requires that an ice-tong like device ("Gardner Wells Tongs" or equivalent) be rigidly connected to the patient's skull by oppositely applied penetrating metal screws. This leaves unsightly holes in the patients head and may introduce a dangerous infection. Furthermore, when traction is applied, very high loads are required because the pull is diffused at every level of the cervical spine, as well as between the skull and the spine, significantly diminishing the effectiveness of the pull at the required level. Not infrequently, the load required will result in the tongs being torn from the skull and through the scalp and face, causing grave injury. While some have attempted to achieve skull traction using various slings and harnesses, they have generally proven both ineffective and equally dangerous.
2. Spreader--It is possible to drill holes into the vertebrae above and below the interspace, and to insert screws therein. A "Casper spreader" device then engages the screws and attempts to push them apart. The screws must pass entirely through the vertebral bodies risking injury to the vital structure or they may tear from the bone risking vertebral fracture and great bodily harm. Unfortunately, placing such screws entirely through the vertebral bodies has been rejected by most doctors as it places the intimately proximate spinal cord at unjustifiable risk. Physicians have further rejected this device since hemorrhaging may follow the removal of the screws.
3. Intervertebral distractors. There are essentially two types of intervertebral distractors. The "Cloward-type" is a ratcheted instrument that opens with a scissor-like movement and which may be placed directly into the interspace, thereby allowing for the highly effective and relatively atraumatic distraction of the interspace. Unfortunately, however, the tips of such distractor engaging the adjacent vertebrae are wide relative to the size of the interspace and tend to sit away from the curved profile of the bone occurring laterally. In general, the available bone grafts, iliac in origin, tend to be almost as wide as the available disc space. Therefore, it is generally not possible to fit both the spreader and the replacement graft into the interspace at the same time. Merely thinning the legs so that they would occupy less space has been tried, and does not work, as even if the remaining leg is sufficiently strong, the thinned profile of the tip tends to cut through and fracture the bone, rendering the device neither safe nor effective.
The second type of intervertebral distractor is typified by the "Karlin Spanner/Distractor", a slender feeler gauge-like device which is turned on edge to hold the space open. Unfortunately, the thin profile tends to cut into the bone, while its placement far laterally results in an asymmetric distraction of the interspace. In general, the disc space is not able to accommodate a spanner on either side and yet allow adequate access for the insertion of the bone graft.
It is therefore apparent that a great need exists for a safe and effective cervical distraction means which ideally would be non-penetrating in its use.